Noninvasive, nuclear imaging techniques can be used to obtain basic and diagnostic information about the physiology and biochemistry of a variety of living subjects including experimental animals, normal humans and patients. These techniques rely on the use of sophisticated imaging instrumentation which is capable of detecting radiation emitted from radiotracers administered to such living subjects. The information obtained can be reconstructed to provide planar and tomographic images which reveal distribution of the radiotracer as a function of time. Use of appropriately designed radiotracers can result in images which contain information on the structure, function and most importantly, the physiology and biochemistry of the subject. Much of this information cannot be obtained by other means. The radiotracers used in these studies are designed to have defined behaviors in vivo which permit the determination of specific information concerning the physiology or biochemistry of the subject or the effects that various diseases or drugs have on the physiology or biochemistry of the subject. Currently, radio-tracers are available for obtaining useful information concerning such things as cardiac function, myocardial blood flow, lung perfusion, liver function, brain blood flow, regional brain glucose and oxygen metabolism.
Compounds can be labeled with either positron or gamma emitting radionuclides. For imaging, the most commonly used positron emitting radionuclides are 11C, 18F, 15O and 13N, all of which are accelerator produced, and have half lives of 20, 110, 2 and 10 min. respectively. Since the half-lives of these radionuclides are so short, it is only feasible to use them at institutions which have an accelerator on site for their production, thus limiting their use. Several gamma emitting radiotracers are available which can be used by essentially any hospital in the U.S. and in most hospitals worldwide. The most widely used of these are 99mTc, 201Tl, and 123I. 123I is particularly useful as a radiotracer for imaging applications because of its ability to form covalent bonds with carbon which, in many cases, are stable in vivo and which have well-understood effects on physiochemical properties of small molecules.
In the past decade, one of the most active areas of nuclear medicine research has been the development of receptor imaging radio-tracers. These tracers bind with high affinity and specificity to selective hormone receptors and neuroreceptors. Successful examples include radiotracers for imagining the following receptor systems: estrogen, muscarinic, dopamine D1 and D2, and opiate.
Currently available chemotherapeutic drugs for treating neoplastic diseases act by disrupting fundamental mechanisms concerned with cell growth, mitotic activity, differentiation and function. The capacity of these drugs to interfere with normal mitosis and cell division in rapidly proliferating tissues is the basis for their therapeutic application as well as toxic properties. As a result, clinical doses of anti-tumor drugs are a compromise between efficacy and toxicity such that therapeutic doses are usually set close to the toxic levels in order to maximize efficacy. In a similar manner, dose selection for clinical evaluation of new anti-tumor drugs is a function of the toxicity of the drug where doses used in Phase II and III trials are often the maximally tolerated doses.
The Ras proteins (Ha-Ras, Ki4a-Ras, Ki4b-Ras and N-Ras) are part of a signaling pathway that links cell surface growth factor receptors to nuclear signals initiating cellular proliferation. Mutated ras genes (Ha-ras, Ki4a-ras, Ki4b-ras and N-ras) are found in many human cancers, including colorectal carcinoma, exocrine pancreatic carcinoma, and myeloid leukemias. The protein products of these genes are defective in their GTPase activity and constitutively transmit a growth stimulatory signal.
At least 3 post-translational modifications are involved with Ras membrane localization, required for normal and oncogenic function, and all 3 modifications occur at the C-terminus of Ras. The Ras C-terminus contains a sequence motif termed a xe2x80x9cCAAXxe2x80x9d or xe2x80x9cCys -Aaa1-Aaa2-Xaaxe2x80x9d box, which, depending on the specific sequence, serves as a signal sequence for the enzymes farnesyl-protein transferase or geranylgeranyl-protein transferase, which catalyze the alkylation of the cysteine residue of the CAAX motif with a C15 or C20 isoprenoid, respectively. (S. Clarke., Ann. Rev. Biochem. 61:355-386 (1992); W. R. Schafer and J. Rine, Ann. Rev. Genetics 30:209-237 (1992)). The Ras protein is one of several proteins that are known to undergo post-translational farnesylation. Other farnesylated proteins include the Ras-related GTP-binding proteins such as Rho, fungal mating factors, the nuclear lamins, and the gamma subunit of transducin. James, et al., J. Biol. Chem. 269, 14182 (1994) have identified a peroxisome associated protein Pxf which is also farnesylated. James, et al., have also suggested that there are farnesylated proteins of unknown structure and function in addition to those listed above.
Inhibition of farnesyl-protein transferase has been shown to block the growth of Ras-transformed cells in soft agar and to modify other aspects of their transformed phenotype. Recently, it has been shown that an inhibitor of farnesyl-protein transferase blocks the growth of ras-dependent tumors in nude mice (N. E. Kohl et al., Proc. Natl. Acad. Sci. U.S.A., 91:9141-9145 (1994) and induces regression of mammary and salivary carcinomas in ras transgenic mice (N. E. Kohl et al., Nature Medicine, 1:792-797 (1995).
Farnesyl transferase inhibitors (FTIs) represent a new pharmacological approach to the treatment of cancer that is mechanism-based and does not rely on a cytotoxic mechanism of action. Ideally, therapeutically effective doses of FTIs will not be limited by cytotoxic side effects and these compounds will have a much larger therapeutic window than currently available anti-tumor drugs.
Farnesyl-protein transferase inhibitors may also be useful for inhibiting other proliferative diseases, both benign and malignant, wherein Ras proteins are aberrantly activated as a result of oncogenic mutation in other genes (i.e., the Ras gene itself is not activated by mutation to an oncogenic form) with said inhibition being accomplished by the administration of an effective amount of the instant composition to a mammal in need of such treatment. For example, a component of NF-1 is a benign proliferative disorder.
Use of farnesyl-protein transferase inhibitors in the prevention of restenosis after percutaneous transluminal coronary angioplasty by inhibiting neointimal formation has recently been described (C. Indolfi et al. Nature medicine, 1:541-545(1995)). It has been disclosed that farnesyl-protein transferase inhibitors may also be useful in the treatment and prevention of polycystic kidney disease (D. L. Schaffner et al. American Journal of Pathology, 142:1051-1060 (1993) and B. Cowley, Jr. et al. FASEB Journal, 2:A3160 (1988)).
It has recently been reported that farnesyl-protein transferase inhibitors are inhibitors of proliferation of vascular smooth muscle cells and are therefore useful in the prevention and therapy of arteriosclerosis and diabetic disturbance of blood vessels (JP H7-112930).
In contrast to cytotoxic chemotherapeutic agents, a more rational approach for estimating clinically-effective doses can be used with FTIs, i.e. it may not be necessary to titrate doses in a patient until toxic effects are observed if that dose is considerably higher than needed to provide the desired effect on tumor growth. Alternatively, if dose-limiting toxicity is observed with the clinical FTI, the dose may not be sufficient to inhibit the enzyme to the extent needed to block tumor growth. Demonstration of clinical efficacy using inhibition of tumor growth or regression in tumor size as an endpoint will require considerable time (weeks to months) and, therefore, dose selection for these trials is very important. Plasma drug concentrations are often used for clinical dose selection, however this endpoint may be a poor surrogate for the drug concentration at the pharmacological target, especially when the site of action is intracellular, such as is the case with farnesyl transferase. PET (Positron Emission Tomography) radiotracers and imaging technology may provide a powerful method for clinical evaluation and dose selection of FTIs. Using a carbon-11 or fluorine-18 labeled radiotracer that enters cells and provides a farnesyl transferase (FPTase) enzyme-specific image in tumors and other tissues, the dose required to saturate FPTase can be determined by the blockade of the PET radiotracer image in humans. The rationale for this approach is as follows: anti-tumor efficacy of FTIs is a consequence of the extent of enzyme inhibition, which in turn is a function of the degree of drug-enzyme occupancy.
Radiolabeled farnesyl-protein transferase inhibitor compounds have recently been described in WO 99/00654, which published on Jan. 7, 1999.
It is, therefore, an object of this invention to develop radiolabeled farnesyl-protein transferase inhibitor compounds that would be useful not only in traditional imaging applications, but would also be useful in assays, both in vitro and in vivo, for labeling the enzyme and for competing with unlabeled farnesyl-protein transferase inhibitors (FTIs). It is a further object of this invention to develop novel assays which comprise such radiolabeled compounds.
It is also the object of this invention to provide for a radiolabeled farnesyl-protein transferase inhibitor compound which is optimized for in vivo imaging and is therefore useful for determining the appropriate clinical doses of an FTI which will be used to assess anti-tumor efficacy in humans.
The present invention is directed toward radiolabeled farnesyl-protein transferase inhibitor compounds, as illustrated by formula I below 
wherein
R is O(CH2)nhalo;
X is a radionuclide comprising 3H, 11C, 18F, 125I, 82Br, 123I, 124I, 131I, 75Br, 76Br, 15O, 13N, 211At, or 77Br; and
n is 1 to 6;
or a pharmaceutically acceptable salt thereof, which are useful to label FPTase in assays, whether cell-based, tissue-based or whole animal. The tracers can also be used in competitive binding assays to obtain information on the interaction of unlabeled FTIs with FPTase.
The present invention is directed toward radiolabeled farnesyl-protein transferase inhibitor compounds of formula I 
wherein
R is O(CH2)nhalo;
X is a radionuclide comprising 3H, 11C, 18F, 125I, 82Br, 123I, 124I, 131I, 75Br, 76Br, 15O, 13N, 211At, or 77Br; and
n is 1 to 6;
or a pharmaceutically acceptable salt thereof
In a second embodiment of the instant invention, the radiolabeled farnesyl-protein transferase inhibitor compounds are illustrated by formula I 
wherein
R is O(CH2)n18F;
X is 127I; and
n is 1 to 6;
or a pharmaceutically acceptable salt thereof.
In a third embodiment of the invention, the radiolabeled farnesyl-protein transferase inhibitor compounds are illustrated by formula II 
wherein
R is O(CH2)nhalo
X is 127I; and
n is 1 to 6;
or a pharmaceutically acceptable salt thereof.
Examples of radiolabeled farnesyl protein transferase inhibiting compounds include the following: 
or a pharmaceutically acceptable salt thereof.
The compounds of the instant invention are useful for labeling farnesyl-protein transferase (FPTase) in assays, whether cell-based, tissue-based or in whole animal. Such radiolabeled compounds can also be used in competitive binding assays to obtain information on the interaction of unlabeled FTIs with FPTase. The in vitro and in vivo assays utilizing the instant radiolabeled compounds are useful in identification of novel compounds that are highly selective inhibitors of FPTase and are therefore useful in the treatment of cancer. The radiolabeled compounds may also be useful in autoradiography and as diagnostic imaging agents.
xe2x80x9cHalogenxe2x80x9d or xe2x80x9chaloxe2x80x9d as used herein means fluoro, chloro, bromo and iodo.
As noted above in the first embodiment of the instant invention, suitable radionuclides, designated as substituent xe2x80x9cXxe2x80x9d, that may be incorporated in the instant compounds include 3H (also written as T), 11C, 18F, 125I, 82Br, 123I, 124I, 131I, 75Br, 76Br, 15O, 13N, 211At, and 77Br. The radionuclide that is incorporated in the instant radiolabeled compounds will depend on the specific analytical or pharmaceutical application of that radiolabeled compound. Thus, for in vitro FPTase labeling and competition assays, inhibitor compounds that incorporate 3H, 125I or 82Br will generally be most useful. For diagnostic imaging agents, inhibitor compounds that incorporate a radionuclide selected from 11C, 18F, 123I, 125I, 124I, 131I, 75Br, 76Br or 77Br are preferred. In certain applications incorporation of a chelating radionuclide such as Tc99m may also be useful. Preferably, in the first embodiment of the instant invention, X comprises 125I, 123I, 124I, 82Br, 75Br, 76Br, SnMe3 and 77Br. Most preferably, in the first embodiment of the instant invention, X comprises 125I or 123I.
In the second and third embodiments of the instant invention, X is designated as 127I since the compounds are radiolabeled with a suitable radionuclide (such as 11C or 18F) at another position.
The labeled farnesyl-protein transferase inhibitor should bind with a high affinity to FPTase. Preferably, the labeled inhibitor has an IC50xe2x89xa610 nM, and most preferably the labeled inhibitor has an IC50xe2x89xa65nM.
Because the FPTase that is interacting with the labeled inhibitor is generally cellular FPTase, the labeled inhibitor of the instant invention must be diffusable across the cell membrane and remain diffusable after binding to FPTase to avoid intracellular accumulation of labeled inhibitor which might contribute to greater assay background noise. Therefore, it is preferred that the labeled inhibitor have a lipo-philicity (partition coefficient) in the range of about 0.5 to about 3.5 and preferably in the range of about 1.0 to about 3.0. It is also preferred that the labeled inhibitor chosen is generally free from nonspecific intracellular interactions that would alter the compounds permeability or effect its FPTase binding affinity. Therefore, while many farnesyl-protein transferase inhibitors have been described that incorporate a thiol moiety, the nonspecific interactions associated with such a moiety disfavor those inhibitors. Similarly, ester prodrugs which exhibit potent intercellular FPTase inhibitory activity only upon conversion to their corresponding acid within the cell are also disfavored because the conversion to the active acid would alter the permeability of the labeled inhibitor.
Radiolabeled FPTase inhibitor compounds, when labeled with the appropriate radionuclide, are potentially useful for diagnostic imaging, basic research, and radiotherapeutic applications. Specific examples of possible diagnostic imaging applications include:
1. Location of primary and metastatic tumors of the pancreas; exocrine tumors;
2. Diagnosis and staging of colorectal carcinoma;
3. Diagnosis and staging of myeloid leukemia;
4. Diagnosis and staging of neurological tumors;
5. Diagnosis and staging of the benign proliferative disorder associated with NF-1;
6. Diagnosis of neointimal formation resulting from percutaneous transluminal coronary angioplasty; and
7. Diagnosis and staging of polycystic kidney disease.
Specific examples of possible radiotherapeutic applications include:
1. Radioimmunoassay of FPTase inhibitors;
2. Radioimmunoassay to determine the concentration of FPTase in a tissue sample; and
3. Autoradiography to determine the distribution of FPTase in a mammal or an organ or tissue sample thereof.
For the use of the instant compounds as diagnostic imaging agents the radiolabeled compounds may be administered to mammals, preferably humans, in a pharmaceutical composition, either alone or, preferably, in combination with pharmaceutically acceptable carriers or diluents, optionally with known adjuvants, such as alum, in a pharmaceutical composition, according to standard pharmaceutical practice. Such compositions can be administered orally or parenterally, including the intravenous, intramuscular, intraperitoneal, subcutaneous, rectal and topical routes of administration. Preferably, administration is intravenous.
For intramuscular, intraperitoneal, subcutaneous and intravenous use, sterile solutions of the labeled compound are usually prepared, and the pH of the solutions should be suitably adjusted and buffered. For intravenous use, the total concentration of solutes should be controlled in order to render the preparation isotonic. For oral use of a diagnostic imaging combination according to this invention, the selected combination or compounds may be administered, for example, in the form of tablets or capsules, or as an aqueous solution or suspension. In the case of tablets for oral use, carriers which are commonly used include lactose and corn starch, and lubricating agents, such as magnesium stearate, are commonly added. For oral administration in capsule form, useful diluents include lactose and dried corn starch. When aqueous suspensions are required for oral use, the active ingredients are combined with emulsifying and suspending agents. If desired, certain sweetening and/or flavoring agents may be added.
Suitable compositions of this invention include aqueous solutions comprising compounds of this invention and pharmacologically acceptable carriers, e.g., saline, at a pH level, e.g., 7.4. The solutions may be introduced into a patient""s bloodstream by local bolus injection.
As used herein, the term xe2x80x9ccompositionxe2x80x9d is intended to encompass a product comprising the specified ingredients in the specific amounts, as well as any product which results, directly or indirectly, from combination of the specific ingredients in the specified amounts.
When a radiolabeled compound according to this invention is administered into a human subject, the amount required for diagnostic imaging will normally be determined by the prescribing physician with the dosage generally varying according to the age, weight, and response of the individual patient, as well as the quantity of emission from the radionuclide. However, in most instances, an effective amount will be the amount of compound sufficient to produce emissions in the range of from about 1-5 mCi.
In one exemplary application, administration occurs in an amount of radiolabeled compound of between about 0.005 xcexcg/kg of body weight to about 50 xcexcg/kg of body weight per day, preferably of between 0.02 xcexcg/kg of body weight to about 3 xcexcg/kg of body weight. A particular analytical dosage that comprises the instant composition includes from about 0.5 xcexcg to about 100 xcexcg of a labeled farnesyl-protein transferase inhibitor. Preferably, the dosage comprises from about 1 xcexcg to about 50 xcexcg of a radiolabeled farnesyl-protein transferase inhibitor.
The following illustrative procedure may be utilized when performing PET imaging studies on patients in the clinic.
The patient is fasted for at least 12 hours allowing water intake ad libitum, and is premedicated with 0.3-0.4 mL Acepromazine injected i.m. on the day of the experiment. A 20 G two inch venous catheter is inserted into the contralateral ulnar vein for radiotracer administration.
The patient is positioned in the PET camera and a tracer dose of [15 O]H2O administered via i.v. catheter. The image thus obtained is used to insure that the patient is positioned correctly to include liver, kidneys, tumors and pancreas. Subsequently, a [11C] radiolabeled farnesyl-protein transferase inhibitor ( less than 20 mCi) is administered via i.v. catheter. Following the acquisition of the total radiotracer image, an infusion is begun of the farnesyl-protein transferase inhibitor which is being clinically evaluated (clinical candidate) at one of three dose rates (0.1, 1 or 10 mpk/day). After infusion for 2.5 hrs, the [11C] radiolabeled farnesyl-protein transferase inhibitor is again injected via the catheter. Images are again acquired for up to 90 min. Within ten minutes of the injection of radiotracer and at the end of the imaging session, 1 ml blood samples are obtained for determining the plasma concentration of the clinical candidate.
For uninhibited distribution of radiotracer, regions of interest (ROIs) are drawn on the reconstructed image includes the tumor, kidney cortex and a region of liver which is removed from the gallbladder images. These regions are used to generate time activity curves obtained in the absence of inhibitor or in the presence of the clinical candidate at the various infusion doses examined. Data are expressed as radioactivity per unit time per unit volume (xcexcCi/cc/mCi injected dose). Inhibition curves are generated from the data obtained in a region of interest obtained starting at 70 min. post-injection of radiotracer. At this time, clearance of non-specific binding has reached steady state. The ID50 values were obtained by curve fitting the dose-rate/inhibition curves with equation iii:                     B        =                                            A              0                        -                          (                                                A                  0                                xc3x97                I                            )                                                          (                                                ID                  50                                +                I                            )                        +            NS                                              (iii)            
where B is the %-Dose/g of radiotracer in tissues for each dose of clinical candidate, A0 is the specifically bound radiotracer in a tissue in the absence of clinical candidate, I is the injected dose of inhibitor, ID50 is the dose of clinical candidate which inhibits 50% of specific radiotracer binding to FPTase, and NS is the amount of non-specifically bond radiotracer.
In the present method, amino acids which are disclosed are identified both by conventional 3 letter and single letter abbreviations as indicated below:
The compounds used in the present method may have asymmetric centers and occur as racemates, racemic mixtures, and as individual diastereomers, with all possible isomers, including optical isomers, being included in the present invention. Unless otherwise specified, named amino acids are understood to have the natural xe2x80x9cLxe2x80x9d stereoconfiguration.
The pharmaceutically acceptable salts of the compounds of this invention include the conventional non-toxic salts of the compounds of this invention as formed, e.g., from non-toxic inorganic or organic acids. For example, such conventional non-toxic salts include those derived from inorganic acids such as hydrochloric, hydrobromic, sulfuric, sulfamic, phosphoric, nitric and the like: and the salts prepared from organic acids such as acetic, propionic, succinic, glycolic, stearic, lactic, malic, tartaric, citric, ascorbic, pamoic, maleic, hydroxymaleic, phenyl-acetic, glutamic, benzoic, salicylic, sulfanilic, 2-acetoxy-benzoic, fumaric, toluenesulfonic, methanesulfonic, ethane disulfonic, oxalic, isethionic, trifluoroacetic and the like.
It is intended that the definition of any substituent or variable (e.g., R, n, etc.) at a particular location in a molecule be independent of its definitions elsewhere in that molecule. It is understood that substituents and substitution patterns on the compounds of the instant invention can be selected by one of ordinary skill in the art to provide compounds that are chemically stable and that can be readily synthesized by techniques known in the art as well as those methods set forth below.
The pharmaceutically acceptable salts of the compounds of this invention can be synthesized from the compounds of this invention which contain a basic moiety by conventional chemical methods. Generally, the salts are prepared by reacting the free base with stoichiometric amounts or with an excess of the desired salt-forming inorganic or organic acid in a suitable solvent or various combinations of solvents.
Abbreviations used in the description of the chemistry and in the Examples that follow are:
The compounds are useful in various pharmaceutically acceptable salt forms. The term xe2x80x9cpharmaceutically acceptable saltxe2x80x9d refers to those salt forms which would be apparent to the pharmaceutical chemist. i.e., those which are substantially non-toxic and which provide the desired pharmacokinetic properties, palatability, absorption, distribution, metabolism or excretion. Other factors, more practical in nature, which are also important in the selection, are cost of the raw materials, ease of crystallization, yield, stability, hygroscopicity and flowability of the resulting bulk drug. Conveniently, pharmaceutical compositions may be prepared from the active ingredients in combination with pharmaceutically acceptable carriers.
Pharmaceutically acceptable salts include conventional non-toxic salts or quarternary ammonium salts formed, e.g., from non-toxic inorganic or organic acids. Non-toxic salts include those derived from inorganic acids such as hydrochloric, hydrobromic, sulfuric, sulfamic, phosphoric, nitric and the like; and the salts prepared from organic acids such as acetic, propionic, succinic, glycolic, stearic, lactic, malic, tartaric, citric, ascorbic, pamoic, sulfanilic, 2-acetoxybenzoic, fumaric, toluene-sulfonic, methanesulfonic, ethane disulfonic, oxalic, isethionic, trifluoroacetic, and the like.
The pharmaceutically acceptable salts of the present invention can be synthesized by conventional chemical methods. Generally, the salts are prepared by reacting the free base or acid with stoichiometric amounts or with an excess of the desired salt-forming inorganic or organic acid or base, in a suitable solvent or solvent combination.
The instant invention is also directed to an assay that measures the competition between a farnesyl transferase inhibitor test compound and a radiolabeled farnesyl transferase inhibitor for binding to farnesyl transferase binding sites in living cells. Such an assay for example would comprise the steps of:
a) culturing monolayers of the cells;
b) exposing a monolayer of cells to growth media containing the radiolabeled farnesyl transferase inhibitor in the presence or absence of the test compound;
c) washing the cells;
d) counting the radiation emitted by the cells; and
e) comparing the radiation emitted by cells exposed to the radiolabeled farnesyl transferase inhibitor and the test compound to the radiation emitted by cells exposed to only the radiolabeled farnesyl transferase inhibitor.
In an embodiment of the above described assay, the monolayer of cells first exposed to growth media containing only the radiolabeled FTI and then, after this pre-exposure, the cells are exposed to growth media containing the test compound. The period of pre-exposure is preferably from about 5 min. to about 1 hour.
Farnesyl-protein transferase inhibitor compounds which incorporate a radionuclide may be prepared by first synthesizing an unlabeled inhibitor that optionally incorporates an iodo or bromo moiety and then exchanging a hydrogen or halogen moiety with an appropriate radionuclide using techniques well known in the art. Syntheses of unlabeled FPTase inhibitors have been generally described in the patent publications cited hereinabove. Syntheses of particular FPTase inhibitors is described below.
These reactions may be employed in a linear sequence to provide the compounds of the invention or they may be used to synthesize fragments which are subsequently joined by the alkylation reactions described in the Schemes.
Synopsis of Schemes 1-4
In Schemes 1-3, variable xe2x80x9cnxe2x80x9d is selected from 1 to 6.
Schemes 1 and 2 shows the synthetic route used to synthesize the iodine-123 and iodine-125 labelled farnesyl transferase inhibitors. As shown in Scheme 1, commercially available 2-methoxy-4-nitroaniline is brominated using NBS/acetonitrile and is then deaminated. The resulting aryl methyl ether (2) is demethylated to give the corresponding phenol (3). Alkylation of the phenol (3) using the appropriate haloalkyl bromide gives the aryl haloalkyl ether (4). Reduction of the aryl nitro group gives the substituted aniline (5) which undergoes reductive amination with Boc-(L)-homoserine lactol to give the protected diamine (6). An acylation/ring closure sequence is used to produce the protected aryl piperazinone (8), and the side chain alcohol is converted to the methyl sulfone (9) in a three step process.
Scheme 2 shows the conversion of this intermediate to the final radioiodinated compounds. Removal of the Boc protecting group and reductive amination gives the cyanobenzyl-imidazoylmethyl piperazinone (11). Conversion of the aryl bromide to the corresponding trimethylstannane (12) is followed by radioiodination which gives the final product (13).
Scheme 3 shows a general synthetic route that could be used to synthesize the fluorine-18 labelled farnesyl transferase inhibitors. The common intermediate for the labelling reaction would be the iodophenol (14) shown. The synthesis of this advanced intermediate would involve synthesizing the appropriate protected phenol following closely the chemistry depicted in Schemes 1 and 2. The protecting group shown in this scheme is the t-butyldiphenylsilyl group because it is compatible with the reaction conditions used in this synthetic scheme. Other protecting groups may be suitable. Commercially available 3,5-dinitrophenol (14) is reduced, diazotized and converted to 3-iodo-5-nitrophenol (16). The silyl protecting group is introduced and the nitro group is reduced to give the silyl protected 3-amino-5-iodophenol (18). The coupling of this intermediate (18) with commercially available Boc-L-methionine sulfone (19) (Bachem) and reduction of the resulting amide gives the diamine (20). This diamine (20) is converted to the piperazinone and coupled with 1-(4-cyanobenzyl)-5-formylimidazole (10), as described in U.S. Pat. No. 5,856,326 (herein incorporated by reference) in Example 42, to give the silyl protected phenylpiperizinone (22). The removal of the silyl protecting group gives the phenol that is the precursor for the fluorine-18 labelling chemistry. This phenol is then treated under basic conditions with the appropriate [18F]bromofluoroalkane, or this phenol is converted to the appropriate alkyl aryl ether containing a leaving group which can be displaced with [18F]fluoride ion.
The incorporation of a [11C]-methyl moiety into a piperazinone containing FPTase inhibitor is shown in Scheme 4. Starting with a hydroxyethyl piperazinone (24), the hydroxy group is converted to the mesylate and then to the trityl protected thiol (25). A deprotection, condensation sequence is used to give the fully assembled compound (26).
The trityl group is removed using TFA/Et3SiH, and the resulting thiol is methylated using [11C]CH3I and oxidized with OXONE to give the final C-11 labelled compound (27). 